A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In an optical lithography apparatus, the patterned beam of radiation is usually of high intensity so as to reduce exposure times and increase throughput. Since the elements of the projection system unavoidably absorb a small percentage of the energy of the beam, these elements will heat up and may thereby distort. Such distortion introduces aberrations into the projection system, distorting the projected image. This problem is known generically as lens heating, although it also affects reflective elements such as mirrors. A variety of methods to correct for lens heating effects are known. These include models to predict lens heating effects and adjustable elements to introduce correcting aberrations into the projection system.